[GitHub] [arrow] nirandaperera commented on a change in pull request #10317: ARROW-12713 [C++] String reverse kernel

[GitBox <gi...@apache.org>]

nirandaperera commented on a change in pull request #10317:
URL: https://github.com/apache/arrow/pull/10317#discussion_r632123654



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File path: cpp/src/arrow/compute/kernels/scalar_string.cc
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@@ -266,6 +266,52 @@ void EnsureLookupTablesFilled() {}
 
 #endif  // ARROW_WITH_UTF8PROC
 
+template <typename Type>
+struct AsciiReverse : StringTransform<Type, AsciiReverse<Type>> {
+  using Base = StringTransform<Type, AsciiReverse<Type>>;
+  using offset_type = typename Base::offset_type;
+
+  bool Transform(const uint8_t* input, offset_type input_string_ncodeunits,
+                 uint8_t* output, offset_type* output_written) {
+    uint8_t utf8_char_found = 0;
+    for (offset_type i = 0; i < input_string_ncodeunits; i++) {
+      // if a utf8 char is found, report to utf8_char_found
+      utf8_char_found |= input[i] & 0x80;

Review comment:
       IMO `ascii_reverse` is slightly different from the other `ascii_*` routines. If there are non-ascii chars present, they can bypass them (ex: lower, upper, etc), but in reverse, we can't do that. Therefore, I added a flag `utf8_char_found` inside the loop, and if any utf8 char is found, the `ascii_reverse` would throw an invalid status. 
   I am not sure, if this check is needed or not. If we can assume that the user passes an 'ascii-only' input, we can simply user `std::reverse_copy`. So, I'd like to get a second opinion on that. 




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